Caffeine consumption by pregnant women can significantly increase the risk of miscarriage, according to new research.

The study, involving more than 1,000 pregnant California women, provides the most convincing evidence to date of such a link, the researchers said.

Research previously indicated an increased risk, but scientists were unsure whether those findings were affected by the fact that women having morning sickness might be less likely to drink caffeinated drinks, such as coffee, and less likely to have miscarriages.

"The relationship between caffeine intake and miscarriage was controversial," said De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente in Oakland, Calif., who led the study. "The question has been whether this association is really due to caffeine itself or something else."

To answer that, Li and his colleagues studied 1,063 women in the San Francisco area early in their pregnancies. They questioned the women in detail about their beverage consumption and whether they were experiencing morning sickness.

Those women who consumed 200 milligrams of caffeine or more a day were about twice as likely to miscarry, the researchers report Monday on the website of the American Journal of Obstetrics and Gynecology. That's about the amount of caffeine in two eight-ounce cups of coffee, five 12-ounce cans of soda or six cups of tea, Li said.

The findings are consistent with those of earlier studies, which have found an increased risk of miscarriage from daily consumption of about 150 to 300 milligrams of caffeine, Li said.

"But we went one step further in determining whether it was the caffeine itself or it was women changing their drinking pattern," he said. "My hope is our study will remove that uncertainty. I think this should put the argument to rest."

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By ROB STEIN

The Washington Post